1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to an improved battery unit configured to prevent short circuit or corrosion due to a contact between a case and an electrode plate or between electrode plates, and a secondary battery employing the battery unit.
2. Description of the Related Art
Lithium batteries are capable of charging and discharging and are classified into nickel-cadmium (Ni—Cd) batteries, nickel-hydride (Ni—MH) batteries and lithium secondary batteries. Specifically, the lithium secondary batteries are rapidly developing in view of their high operating voltage of 3.6 V or higher, which is approximately 3 times that of the nickel-cadmium (Ni—Cd) batteries or nickel-hydride (Ni—MH) batteries, and their excellent energy density per unit weight.
The lithium secondary batteries can be classified into liquid electrolyte batteries and solid electrolyte batteries according to the electrolyte used. In general, batteries using a liquid electrolyte are referred to as lithium-ion batteries and batteries using a polymeric electrolyte are referred to as lithium polymer batteries. The lithium secondary batteries can be manufactured in various shapes, typically in cylindrical and rectangular shapes, which are mainly used in lithium ion batteries. Lithium polymer batteries have lately attracted attention since they are manufactured by a flexible material rendering them to be shaped freely. Also, since the lithium polymer batteries have good safety characteristics and are lightweight, they can be advantageously used in miniaturized and lightweight portable electronic devices.
Referring to FIG. 1, a secondary battery 10 includes a battery unit 14 having a positive electrode plate 11 and a negative electrode plate 12. A separator 13 is disposed between the positive and negative electrode plates 11 and 12 and electrically insulates the positive and negative electrode plates 11 and 12. A positive electrode tab 15 and a negative electrode tab 16 are respectively drawn out from the respective electrode plates 11 and 12 of the battery unit 14. A positive electrode terminal 17 and a negative electrode terminal 18 are electrically connected to the positive and negative tabs 15 and 16. A case 19 includes a space 100 in which the battery unit 14 is housed. The case 19 is shaped as a pouch and includes an upper case 19a and a lower case 19b. 
The above-described secondary battery 10 is constructed such that the battery unit 14 having the positive electrode plate 11, the separator 13 and the negative electrode plate 12 are sequentially stacked and mounted in the pouch-shaped case 19. In order to fabricate the stack-type battery unit 14, the positive and negative plates 11 and 12 with the separator 13 interposed therebetween must be laminated several times to complete a unit cell, and a plurality of unit cells must be stacked. This approach, however, limits continuous mass production. Also, in the course of cutting the unit cell to be stacked, a short circuit may occur due to burrs generated at ends of the positive and negative electrode plates 11 and 12, resulting in severe deterioration of the completed battery.
To overcome the shortcomings of the above mentioned stack-type battery unit, a fabrication method has been recently adopted, in which a positive electrode plate 11, a negative electrode plate 12, and a separator 13, interposed between the positive and negative plates, are each formed into a foil, and the positive electrode plate 11, the separator 13, and the negative electrode plate 12 are sequentially arranged and wound to then be mounted in the pouch-shaped case 19. FIG. 2 is a front view of an electrode plate 20 of such a battery unit, and FIG. 3 is a rear view of the electrode plate 20.
Referring to FIGS. 2 and 3, the electrode plate 20 includes a current collector 21 and electrode sheets formed on both surfaces of the current collector 21. The electrode sheets are a front electrode sheet 22 and a rear electrode sheet 23 formed on the front and rear surfaces of the current collector 21, respectively. In the current collector 21, an area is provided where the front and rear electrode sheets 22 and 23 are not coated, called an uncoated area. The uncoated area includes a front electrode uncoated area 24 and a rear electrode uncoated area 25 formed on the front and rear surfaces of the current collector 21, respectively. An electrode tab 26 is welded to the rear electrode uncoated area 25. A portion of the electrode tab 26 is wrapped with a protective tape 27. One or more insulating tapes 28 are adhered to a boundary between the front electrode sheet 22 and the front electrode uncoated area 24 and between the rear electrode sheet 23 and the rear electrode uncoated area 25, respectively.
In the above-described electrode plate 20, the insulating tapes 28 have the same width as the width of the current collector 21, and are selectively adhered to the front or rear surfaces of the current collector 21.
The insulating tapes 28 are required to protect a portion where the electrode plate 20 and the electrode tab 26 are electrically connected to each other and prevent short circuits due to a contact between opposite electrode plates. The mechanism to prevent short circuits is formed at a portion where the electrode tab 26 is installed, and at boundaries between the front electrode sheet 22 and the front electrode uncoated area 24 and between the rear electrode sheet 23 and the rear electrode uncoated area 25. Thus, short circuits due to a contact between electrode plates 11 and 12 can be prevented during operation of the secondary battery. The conventional secondary battery poses some limitations.
The width of the insulating tape 28 is the same as that of the current collector 21, and the insulating tape 28 is not formed in the front electrode uncoated area 24. Accordingly, if a separator 13 interposed between electrode plates 11 and 12 undergoes deformation such as shrinkage, during operation of the battery, the electrode plates 11 and 12 may contact each other, resulting in a short circuit.
Also, in the pouch-shaped battery case 19, having a multi-layer structure, a short circuit may occur at a medium layer made of metal that becomes exposed when an inner layer as an insulation layer is damaged and causes a contact between two electrode plates 11 and 12 of a wound battery unit. Also, if there is a contact between an electrode plate and a medium layer of the pouch-shaped battery case 19, for example, if a current collector made of a copper foil is electrically connected to a medium layer made of an aluminum foil, the pouch-shaped battery case 19 gradually corrodes depending on a difference in ionization level between the current collector and the medium layer. Therefore, there is a demand for secondary batteries designed to have a mechanism to prevent short circuits.